The invention relates to a process for coating metal substrates, in particular steel or aluminum sheet in strip form with a coating of metal or metal alloy, including cleaning, degreasing and activating the substrate as well as an apparatus for carrying out the process, comprising a material inlet lock section, a vacuum section including an activating section and a coating section for the application of the coating as well as a material exit lock section.
The coating of metals with other metals or metal alloys is a widely used method in order to render base metals better suitable for various purposes. Thus, for example, increased attempts have been made lately to employ aluminum alloys as materials for vehicle bodies of motor vehicles in order to save weight. Essentially for that purpose a self-hardening aluminum alloy composed of 95.2% Al, 4.5% Mg and 0.3% Mn can be used for the interior components and a secondarily hardenable and flow line free aluminum alloy composed of 98.4% Al, 0.4% Mg and 1.2% Si for body shell components. Even though Al-sheet metal of the secondarily hardenable alloy offers strength characteristics which are comparable to steel sheet, such sheet metal is nevertheless employed nowadays only rarely for motorcar construction inspite of a 45% weight saving. The reasons, therefore, are the poor phosphatising properties and poor enamelling properties as compared with steel and the poor spot-welding properties, due to the high electrical resistance resulting from the oxide layer, for which reason the aforesaid aluminum sheets cannot be processed in existing production lines.
Accordingly, attempts are being made increasingly to avoid these drawbacks by the application of a zinc coating. Initially this was attempted electrolytically. In order to attain a strongly adhering coating, it is then necessary to remove the highly reactive oxide layer prior to coating. In JP-B-90050989 and in JP-1047895 processes that purpose are described in which, by means of a dipping treatment, (zincate process) the oxide layer is removed and simultaneously a thin (0,5 .mu.m) zinc layer is applied. The zinc coating proper takes place subsequently electrolytically. JP-A-03207885 and JP-A-314668 disclose the electrolytic application of a zinc coating in baths containing fluoride or chloride. However, those processes suffer from the disadvantage that they involve high energy expenditure, cause effluent problems, that zinc sludge is produced which must be disposed of and that they can, in may cases, be carried out only as a multiple-stage process.
More recently, coating by means of vacuum vapor coating has been gaining increasing importance for being an environmentally friendly process. The aforesaid disadvantages can be avoided by vapor deposition in a vacuum. In order to attain strongly adhering coatings in such processes, the metals substrate is preferably subjected to a pre-treatment in a vacuum comprising generally a pre-treatment of the strip to temperatures of 200.degree. to 300.degree. C. Where aluminum is the substrate material, higher pre-heating temperatures of about 300.degree. to 400.degree. C. are necessary because of the oxide layer. With steel, specifically when applying zinc coating, even higher temperatures of about 400.degree. to 500.degree. C. are necessary. In order to avoid re-evaporation, the substrate must then be cooled prior to coating, which from a process point of view involves substantial disadvantages. Moreover, in the case of hardenable aluminum materials, a deterioration of the mechanical properties is experienced.